[BUG] Can't parse schema with external $ref

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

openapi-generator-cli can't parse schema with external $ref.

[main] WARN  io.swagger.v3.parser.OpenAPIV3Parser - Exception while resolving:
java.lang.NullPointerException: null
        at io.swagger.v3.parser.util.OpenAPIDeserializer.getSchema(OpenAPIDeserializer.java:2467)
        at io.swagger.v3.parser.util.OpenAPIDeserializer.getSchema(OpenAPIDeserializer.java:2349)
        at io.swagger.v3.parser.ResolverCache.loadRef(ResolverCache.java:152)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processRefToExternalSchema(ExternalRefProcessor.java:75)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processRefSchema(ExternalRefProcessor.java:921)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processSchema(ExternalRefProcessor.java:200)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processProperties(ExternalRefProcessor.java:223)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processProperties(ExternalRefProcessor.java:230)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processRefToExternalSchema(ExternalRefProcessor.java:157)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processRefToExternalSchema(ExternalRefProcessor.java:119)
        at io.swagger.v3.parser.processors.SchemaProcessor.processReferenceSchema(SchemaProcessor.java:214)
        at io.swagger.v3.parser.processors.SchemaProcessor.processArraySchema(SchemaProcessor.java:198)
        at io.swagger.v3.parser.processors.SchemaProcessor.processSchemaType(SchemaProcessor.java:47)
        at io.swagger.v3.parser.processors.SchemaProcessor.processSchema(SchemaProcessor.java:39)
        at io.swagger.v3.parser.processors.ResponseProcessor.processResponse(ResponseProcessor.java:52)
        at io.swagger.v3.parser.processors.OperationProcessor.processOperation(OperationProcessor.java:67)
        at io.swagger.v3.parser.processors.PathsProcessor.processPaths(PathsProcessor.java:84)
        at io.swagger.v3.parser.OpenAPIResolver.resolve(OpenAPIResolver.java:49)
        at io.swagger.v3.parser.OpenAPIV3Parser.resolve(OpenAPIV3Parser.java:175)
        at io.swagger.v3.parser.OpenAPIV3Parser.readContents(OpenAPIV3Parser.java:154)
        at io.swagger.v3.parser.OpenAPIV3Parser.readLocation(OpenAPIV3Parser.java:89)
        at io.swagger.parser.OpenAPIParser.readLocation(OpenAPIParser.java:16)
        at org.openapitools.codegen.config.CodegenConfigurator.toContext(CodegenConfigurator.java:523)
        at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:573)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:432)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
[main] WARN  o.o.codegen.utils.ModelUtils - Failed to get the schema name: https://raw.githubusercontent.com/beatcracker/toptout/master/schema/toptout.schema.json
[main] WARN  o.o.codegen.utils.ModelUtils - Failed to get the schema name: https://raw.githubusercontent.com/beatcracker/toptout/master/schema/toptout.schema.json
[main] WARN  o.o.codegen.utils.ModelUtils - Failed to get the schema name: https://raw.githubusercontent.com/beatcracker/toptout/master/schema/toptout.schema.json
[main] WARN  o.o.codegen.utils.ModelUtils - Failed to get the schema name: https://raw.githubusercontent.com/beatcracker/toptout/master/schema/toptout.schema.json
[main] WARN  o.o.codegen.utils.ModelUtils - Failed to get the schema name: https://raw.githubusercontent.com/beatcracker/toptout/master/schema/toptout.schema.json
[main] WARN  o.o.codegen.utils.ModelUtils - Failed to get the schema name: https://raw.githubusercontent.com/beatcracker/toptout/master/schema/toptout.schema.json
Exception in thread "main" org.openapitools.codegen.SpecValidationException: There were issues with the specification. The option can be disabled via validateSpec (Maven/Gradle) or --skip-validate-spec (CLI).
 | Error count: 1, Warning count: 2
Errors:
        -null
Warnings:
        -null

        at org.openapitools.codegen.config.CodegenConfigurator.toContext(CodegenConfigurator.java:546)
        at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:573)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:432)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)

openapi-generator version

5.1.0-SNAPSHOT

OpenAPI declaration file content or url

• OpenAPI spec: openapi.yaml
• $ref: toptout.schema.json

Generation Details

docker run --rm openapitools/openapi-generator-cli generate -i https://raw.githubusercontent.com/beatcracker/toptout/master/docs/swagger/openapi.yaml -g powershell

Steps to reproduce

Just run the command above.

Related issues/PRs

#1976

Suggest a fix

Reproduce the failure with the listed Docker command and the linked openapi.yaml and toptout.schema.json. Start by reading the OpenAPIV3Parser, ResolverCache, and ExternalRefProcessor stack entries. Done means the external $ref is parsed without the NullPointerException and specification validation succeeds.